The present invention relates to a gantry with an ion-optical system according to the preamble of claim 1.
Such an ion-optical system for a gantry encloses a first bending magnet with a bending angle as known from U.S. Pat. No. 4,870,287 of 90xc2x0 for bending a proton beam off an axis of rotation of said gantry. Further a second bending magnet with a bending angle identical to the bending angle of the first bending magnet bends said ion beam parallel to said axis of said gantry. Finally, a third bending magnet with a bending angle of 90xc2x0 according to the above mentioned prior art bends said ion beam toward an intersection of said ion beam with said axis of rotation of said gantry. This intersection is called isocentre.
From U.S. Pat. No. 4,870,287 is further known, that between the first and the second bending magnet two quadrupole magnets are positioned. Also between the second and the third bending magnets two other quadrupole magnets are positioned. The disadvantage of such a gantry is, however, that if a non-symmetric ion beam is introduced to the gantry input from a fixed transfer line the beam transport within such a gantry having only four quadrupoles becomes dependent on the angle of gantry rotation, wherein the non-symmetric beam is ment as a beam having different emittances in vertical and horizontal planes.
Theoretical studies of medical synchrotrons as well as the measurements at existing facilities have shown that the slowly extracted beams do have the above mentioned different emittances in horizontal and vertical planes. This complicates the matching of the fixed transfer line to the rotating gantry. The input beam parameters in the horizontal and vertical planes of the gantry become a function of the angle of gantry rotation and this dependence, unless special precautions are applied, is transformed also to the beam parameters of the gantry exit.
To overcome these disadvantages a special matching section, called a xe2x80x9crotatorxe2x80x9d comprising additionally to the quadrupole magnets within the gantry a plurality of other quadrupole magnets was proposed by M. Benedikt and C. Carli, xe2x80x9cMatching to gantries for medical synchrotronsxe2x80x9d, Particle Accelerator Conference PAC ""97, Vancouver 1997. The rotator is positioned upstream the gantry within the fixed transfer line.
The rotator provides a universal method allowing to match the rotating gantries to the fixed transfer lines without applying any specific ion-optical constraints upon the gantries. On the other hand, it occupies about 10 m of extra length of the transfer line, which is a disadvantage for design of extremely compact medical accelerator complexes fitting to the hospitals.
In addition the entire rotator has to be rotated synchronously with the gantry, which requires an extra equipment for extremely precise mechanical rotation.
Therefore it is the object of the present invention to save space and costs and to avoid such a rotator so that a gantry rotation independent transport of non-symmetric ion beams is possible. Thus, the ion-optical settings should make the beam parameters at the gantry exit independent from the angle of gantry rotation even if the beam enters the gantry with different emittance in horizontal and vertical planes.
This object is achieved by the subject matter of independent claim 1. Features of preferred embodiments are enclosed in dependent claims, depending on claim 1.
Therefore the gantry with an ion-optical system further comprises:
a horizontal scanner magnet positioned upstream said third bending magnet for horizontal scanning of said ion beam in a plane perpendicular to the beam direction;
a vertical scanning magnet positioned upstream said third bending magnet for vertical scanning of said ion beam in a plane perpendicular to the beam direction,
at least six quadrupole magnets with adjustable excitation positioned downstream the said first bending magnet and upstream said scanner magnets, wherein the quadrupole magnets provide:
a fully achromatic beam transport from the gantry entrance to the isocentre;
a control of the size of said ion beam in the isocentre according to a pre-defined beam-size pattern; and
the size of said ion beam in the isocentre and the spot-shape of said ion beam in the isocentre which is independent from the angle of gantry rotation, wherein the gantry can be rotated to any angle between 0xc2x0 and 360xc2x0 with respect to a fixed beam transfer line connecting an accelerator with the gantry and wherein said ion beam coming from said fixed beam transfer line and entering said gantry has different emittances in the horizontal and vertical planes of said fixed beam transfer line.
It is a well-known fact that beams of ions (typically 1xe2x89xa6Zxe2x89xa68) have favourable physical and biological properties for their use in cancer-therapy. The most appropriate beam delivery technique, in particular for ions heavier than protons, is a so-called active beam scannig comprising an energy variation from the accelerator and lateral intensity-controlled raster scanning according to the characterising portion of claim 1. In contrast to a passive beam delivery the active scanning systems aim to deliver a narrow pencil-like beam with a variable spot size to the patient and to scan it over the treatment area.
Forming and preservation of the pencil-like beam by the beam transport system is of crucial importance in this case for the ion-optical system of the gantry. The dose-to-target conformity can further be optimised if the beam can enter the patient from any direction. This task is performed by said gantry which is rotated around the horizontal axis with respect to the room, coordinate system. The combination of the pencil-beam scanning with a rotating gantry brings about special additional ion-optical problems.
The beam is described at the exit of the transfer line, at the entrance of the gantry, and at the gantry isocentre by its sigma-matrices "sgr"(0), "sgr"(1), and "sgr"(2), respectively, where the sigma-matrices have, in general, a form:                               σ          ⁢                      xe2x80x83                    ⁢                      (            i            )                          =                                                            σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    11                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    12                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    13                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    14                                                                                                        σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    21                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    22                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    23                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    24                                                                                                        σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    31                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    32                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    33                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    34                                                                                                        σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    41                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    42                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    43                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    i                    )                                    44                                                                                        (        1        )            
where i=0, 1, 2 and the individual matrix-terms have their usual meaning. The sigma-matrix is a real positive definite, and symmetric matrix. The square roots of the diagonal terms of the sigma-matrix are a measure of the beam size in x, xxe2x80x2, y and yxe2x80x2 coordinates, where [x, xxe2x80x2, y, yxe2x80x2] is a four-dimensional phase space in which the beam occupies a volume inside a four-dimensional ellipsoid characterised by the sigma-matrix. The off-diagonal terms determine the orientation of the ellipsoid in the phase space. At the exit of the transfer line, a so-called uncoupled beam is expected, i.e. there is no correlation between the two transverse phase spaces [x, xxe2x80x2] and [y, yxe2x80x2]. In such a case, the elements of the sigma-matrix coupling the horizontal and vertical phase vanish. Taking into account these properties, the sigma-matrix of the beam at the exit of the transfer line can be written in a simplified form:                               σ          ⁢                      xe2x80x83                    ⁢                      (            0            )                          =                                                            σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    0                    )                                    11                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    0                    )                                    12                                                                    0                                      0                                                                          σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    0                    )                                    12                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    0                    )                                    22                                                                    0                                      0                                                          0                                      0                                                      σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    0                    )                                    33                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    0                    )                                    34                                                                                        0                                      0                                                      σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    0                    )                                    34                                                                                    σ                ⁢                                  xe2x80x83                                ⁢                                                      (                    0                    )                                    44                                                                                        (        2        )            
If the gantry is rotated with respect to the fixed transfer line by an angle xcex1, the sigma-matrix of the beam at the gantry entrance, "sgr"(1), will be given by the transformation:
"sgr"(1)=Mxcex1xc2x7"sgr"(0)xc2x7Mxcex1Txe2x80x83xe2x80x83(3)
where Mxcex1T is a transpose matrix to Mxcex1 describing the rotation of the coordinate system by an angle a and having a form                               M          ⁢                      xe2x80x83                    ⁢          α                =                                                            cos                ⁢                                  xe2x80x83                                ⁢                α                                                    0                                                      sin                ⁢                                  xe2x80x83                                ⁢                α                                                    0                                                          0                                                      cos                ⁢                                  xe2x80x83                                ⁢                α                                                    0                                                      sin                ⁢                                  xe2x80x83                                ⁢                α                                                                                                          -                  sin                                ⁢                                  xe2x80x83                                ⁢                α                                                    0                                                      cos                ⁢                                  xe2x80x83                                ⁢                α                                                    0                                                          0                                                                        -                  sin                                ⁢                                  xe2x80x83                                ⁢                α                                                    0                                                      cos                ⁢                                  xe2x80x83                                ⁢                α                                                                        (        4        )            
The beam transport system of the gantry is characterised by its transfer matrix MGAN:                               M          GAN                =                                                            r                11                                                                    r                12                                                    0                                      0                                                                          r                21                                                                    r                22                                                    0                                      0                                                          0                                      0                                                      r                33                                                                    r                34                                                                        0                                      0                                                      r                43                                                                    r                44                                                                        (        5        )            
The transfer matrix MGAN must fulfil a condition detMGAN=1 (6). It is already assumed in eq. (5) that there is no coupling between horizontal and vertical planes in the gantry optics itself. The pertinent terms have been set to zero. In addition, the beam transport system of the gantry is assumed to be achromatic in the gantry isocentre. (DISO=0, Dxe2x80x2ISO=0 (7, 8), where DISO is the dispersion function at the gantry isocentre), which allows to write the transfer matrix MGAN in a 4xc3x974 form without the terms related to the dispersion function. The sigma-matrix of the beam at the gantry isocentre, "sgr"(2), is given by the relation:
"sgr"(2)=MGANxc2x7"sgr"(1)xc2x7MGANTxe2x80x83xe2x80x83(9)
The achromatic beam transport in the gantry must also be independent from the angle of gantry rotation. This can be achieved, if the dispersion function and its derivative are set to zero at the gantry entrance. This translates to the ion-optical constraints upon the transfer line to form a dispersion-free region at its exit.
Not all beam parameters at the gantry isocentre must be independent from the angle of the gantry rotation. For tumour irradiation, it is enough to achieve an angular independence for the beam size in both transverse planes and for the shape of the beam spot. A round beam spot is required. In the sigma-matrix formalism language, these requirements can be written as:
"sgr"(2)11="sgr"(2)33xe2x89xa0f(xcex1) AND "sgr"(2)13=0xe2x89xa0f(xcex1)xe2x80x83xe2x80x83(10)
In addition, the beam size must be adjustable from 4 to 10 mm diameter.
After listing the basic assumptions, we can proceed to investigate a dependence of the beam parameters at the gantry isocentre on the angle of gantry rotation. It is convenient to combine eqs. (3) and (9) in order to express the overall transformation from the exit of the transfer line to the gantry isocentre.
"sgr"(2)=MGANxc2x7"sgr"(1)xc2x7MTGAN=MGANxc2x7Mxcex1xc2x7"sgr"(0)xc2x7MTxcex1xc2x7MTGAN
xe2x80x83=MOVERxc2x7"sgr"(0)xc2x7MTOVERxe2x80x83xe2x80x83(11)
where MOVER is the overall transfer matrix from the exit of the transfer line to the gantry isocentre given by a relation:
MOVER=MGANxc2x7Mxcex1xe2x80x83xe2x80x83(12)
Because the matrix Mxcex1 contains the terms depending on the angle of gantry rotation xcex1 (see eq. (4)), these terms xe2x80x98penetratexe2x80x99 also to the sigma-matrix of the beam in the gantry isocentre "sgr"(2) through the transformation (11). One gets by performing the matrix multiplication (11):
"sgr"(2)11=r211xc2x7["sgr"(0)11 cos2xcex1+
"sgr"(0)33 sin2xcex1]+2r11r12"sgr"
(0)12 cos2xcex1++r212xc2x7["sgr"(0)22 cos2xcex1+"sgr"
(0)44 sin2xcex1]+2r11r12"sgr"(0)34 sin2xcex1xe2x80x83xe2x80x83(13)
It is interesting to find out that eliminating the xcex1-containing terms in eq. (13) is feasible and that there are even several ways to do so. Equation (13) becomes:
"sgr"(2)11=r212xc2x7"sgr"(0)22xe2x89xa0f(xcex1) if 
r11=0 AND "sgr"(0)22="sgr"(0)44xe2x80x83xe2x80x83(14)
or
"sgr"(2)11=r211xc2x7"sgr"(0)11xe2x89xa0f(xcex1) if 
r12=0 AND "sgr"(0)11="sgr"(0)33xe2x80x83xe2x80x83(15)
An identical set of constraints can be obtained in the vertical plane of the gantry:
{r33=0 AND "sgr"(0)22="sgr"(0)44} OR 
{r34=0 AND "sgr"(0)11="sgr"(0)33}xe2x80x83xe2x80x83(16)
The xy correlation term "sgr"(2)13 will be:
"sgr"(2)13=sin xcex1 cos xcex1xc2x7[r11r33("sgr"(0)33xe2x88x92"sgr"
(0)11)+r11r34("sgr"(0)34xe2x88x92"sgr"(0)12)++
r12r34("sgr"(0)44xe2x88x92"sgr"(0)22)+
r12r33("sgr"(0)34xe2x88x92"sgr"(0)12)]xe2x80x83xe2x80x83(17)
The condition "sgr"(2)13=0 xe2x89xa0f (xcex1) is satisfied if the following constraints are fulfilled:
{r11=0 AND r33=0 AND "sgr"(0)22="sgr"
(0)44} OR {r12=0 AND r34=0 AND 
"sgr"(0)11"sgr"(0)33}
The logical relations between the above ion-optical constraints are sketched in Tab. 1.